It is well recognized that steel fiber reinforced concrete composites exhibit improved resistance to fracture and impact loads. Both fracture and impact resistance are primarily governed by the toughness characteristics of the material defined by its energy-absorption capacity. Toughness can be measured by carrying out tests involving direct tension, compression, or flexure. However, flexural tests are favored for measurement of toughness because of their simplicity and also their close representation of the stress conditions under field conditions. The test procedures for the measurement of toughness indexes given in codes of practice such as ASTM C 1018, JCI-SF4, JSCE-SF4, and ACI 544 help to obtain information on the qualitative performance of different materials and mix designs. Little information has been reported on the toughness characteristics of slurry-infiltrated fibrous concrete (SIFCON), which is basically a material formed by infiltrating a preplaced "fiber stack" with a cement slurry. This paper describes the details of toughness tests carried out on SIFCON at the Structural Engineering Research Centre, Madras, India, and summarizes the results of the investigations.

The effectiveness of steel fibers as shear reinforcement to replace and/or augment conventional stirrups in concrete beams with flexural reinforcement has been demonstrated by Batson, et al. (1972), J. Craig (1984), and other researchers. The current thinking within ACI Committee 544 is to adjust the limiting values of the empirical equations for shear design in ACI 318. However, a rational basis for the design or analysis of steel fibers as shear reinforcement has not been developed. Possible approaches can be based on the plasticity of concrete, Chen (1978) and Nielsen (1984); and limit state analysis and the modified compression field theory, Marti (1986) and Collins (1984). Test data for flexural reinforced concrete beams using steel fibers as the shear reinforcement match the lower bound solution for the shear strength as a function of the shear span-depth ratio, based on limit states analysis of concrete by Nielsen and Braestrup (1978) and Kemp and Al-Safi (1981). The test data agree well with the theoretical predicted strength, assuming the steel fiber concrete is rigid-plastic with a modified Coulomb failure criterion for the yield condition, no tensile strength, and the compressive strength is the effective compressive strength. The plasticity assumption for steel fiber reinforced concrete is supported by research reported on its torsional strength by Narayanan et al. (1979 and 1983), in which the torsional strength was best predicted by the Nadai's "sand heap" plastic model for a variety of steel fiber volumes in the concrete. The random distribution of the steel fibers at relatively close spacing provides a very ductile mode of failure that is in good agreement with strength theories based on plasticity theory and limit states. The initial test results suggest that a rational design procedure for the shear strength of steel fiber concrete can be based on a modified compression field theory that will be accepted by design engineers. This paper briefly reviews the current thinking on shear design of beams using steel fibers as the shear reinforcement, plastic material response of steel fiber concrete, and test data that agrees with the plasticity properties and limit theorems proposed by Nielsen and Braestrup and by Kemp and Al-Safi for reinforced and prestressed concrete beams without shear reinforcement.

Relatively low-cost and energy-efficient materials with desirable short-term mechanical properties can be constructed using cellulose fibers as cement reinforcement. There are, however, concerns regarding the long-term performance of cellulose fiber reinforced cement composites; some cellulose fibers tend to disintegrate in the alkaline environment of cement. The growth of cement hydration products within the hollow cellulose fibers may also lead to excessive fiber-to-matrix bonding and brittle failure after exposure to natural weathering. This paper presents the results of an experimental study concerned with the long-term performance of cellulose fiber reinforced cement composites. Cellulose fiber reinforced cement composites were investigated, using accelerated weathering conditions representing repeated wetting and drying of materials in outside exposure conditions. The cement composites considered in this investigation incorporated 2 percent mass fractions of kraft pulp. Comprehensive replicated flexural test data were generated for various test ages at different wetting-drying cycles and were analyzed statistically. The analysis of variance and multiple comparison techniques were employed to derive reliable conclusions regarding the effect of accelerated wetting-drying cycles on flexural strength and toughness characteristics of cellulose fiber reinforced cement composites. The results generated in this study showed, at 95 percent level of confidence, that accelerated aging under repeated wetting-drying cycles had negligible effects on flexural strength, but led to reduced toughness and embrittlement of cellulose fiber reinforced cement composites.

An innovative technique for reinforcing concrete to achieve extremely high flexural strengths has been developed. This technique utilizes a steel fiber mat instead of short, discrete steel fibers. The mat configuration is preplaced for infiltration with a concrete slurry to yield a composite with flexural strengths approaching ten times that of conventional concrete. Applications include high-performance bridge decks, earthquake-resistant structures, nuclear waste containment, military applications, and other innovative uses in which flexural strength is at a premium. Stainless steel mats or other advanced alloys can be provided where corrosion resistance or high temperature strength are required.

Reports the results of an investigation on the strength and ductility of fiber reinforced high-strength concrete under direct shear. Both experimental and modeling studies were performed. In the experimental study, fiber reinforced high-strength concrete pushoff specimens were tested. Two fiber types, polypropylene and steel, were used with or without conventional stirrups. An existing model was further developed and used in the analytical prediction of the shear stress-strain relationships for these specimens. In general, fibers proved to be more effective in high-strength concrete than in normal strength concrete, increasing both ultimate load and overall ductility. This is attributed to the improved bond characteristics associated with the use of fibers in conjunction with high-strength concrete. For the specimens with steel fibers, significant increases in ultimate load and ductility were observed. With polypropylene fibers, a lower increase in ultimate load was obtained when compared to the increase due to steel fibers. Ductility of the polypropylene fiber reinforced specimens was greater than that of the steel fiber reinforced specimens. In the tests involving the combination of fibers and conventional stirrups, slight increases in ultimate load and major improvements in ductility were observed when compared to the values for plain concrete specimens with conventional stirrups. In general, good agreement between the model and the test results was found.